1. Field of the Invention
The present invention relates to a color toner, a developer and a developing device, and an image forming apparatus having the developing device.
2. Description of the Related Art
An image forming apparatus employing an electrophotographic system for forming images by means of electrophotography is capable of forming high-quality images with ease, and is thus widely used for example as a copier, a printer, a facsimile machine, and a multifunctional peripheral. In particular, an image forming apparatus that enables full-color printing is springing into wide use.
An electrophotographic image forming apparatus (hereafter referred to simply as “image forming apparatus”) includes, for example, a photoreceptor, a charging section, an exposure section, a developing section, a transfer section, and a fixing section. The image forming apparatus performs a charging process, an exposure process, a development process, a transfer process, and a fixing process with use of the photoreceptor and the aforementioned sections thereby to form images on a recoding medium.
In the charging process, the surface of the photoreceptor is uniformly charged by the charging section. In the exposure process, the photoreceptor in a charged state is exposed to light by the exposure section to form an electrostatic latent image on the surface of the photoreceptor. In the development process, charge-bearing toner is caused to adhere to the electrostatic latent image formed on the surface of the photoreceptor by the developing section thereby to form a visual image on the surface of the photoreceptor. In the transfer process, the visual image formed on the surface of the photoreceptor is transferred onto a recording medium such as paper or a sheet by the transfer section. Note that, in full-color printing operation, there may be a case where the visual image formed on the surface of the photoreceptor is transferred onto an intermediate transfer medium first, and is then transferred onto a recording medium via the intermediate transfer medium. In the fixing process, the transferred visual image is fixed onto a recording medium under application of for example heat and pressure by the fixing section. The development process includes a charge application step to apply electric charge to toner contained in a developer.
In the image forming apparatus, full-color printing can be achieved by using, as a developer, toners of the three primary colors, to be specific, toners of three colors: an yellow-color toner; a magenta-color toner; and a cyan-color toner, or toners of four colors: the aforementioned three color toners and a black-color toner. The charging process, the exposure process, the development process, and the transfer process are carried out for the toner of each color on an individual basis, whereupon a visual image consisting of the toners of a plurality of colors is formed on a surface of a recording medium. Then, in the fixing process, after the plural-color toners are melted and mixed so as to achieve color blending, the visual image is fixed onto the recording medium. In this way, a full-color image can be formed.
As the developer for forming the visual image, there are two types: a two-component developer composed of carrier and toner; and a one-component developer composed solely of toner. In the charge application step, in the case of using the two-component developer, it is possible to apply electric charge to the toner by producing friction between the toner and the carrier acting as a charge applying portion. On the other hand, in the case of using the one-component developer, it is possible to apply electric charge to the toner by bringing the toner into pressure-contact with a layer-thickness regulating member acting as a charge applying portion.
In the charge application step, smooth charge movement between toner particles and smooth charge movement between the toner and the charge applying portion as well, and stable control of the amount of charge applied to the toner are of importance to form a high-quality image. In particular, a color toner which contains an organic pigment as a colorant exhibits higher resistance than a black toner does, and therefore charge movement cannot be readily produced therein. Accordingly, stable control of the amount of charge applied to the toner is especially important if the color toner is employed.
As a technique to achieve stable control of the amount of charge applied to the toner, Japanese Unexamined Patent Publication JP-A 4-452 (13992) discloses external addition of titanium oxide subjected to surface treatment with metal salt of fatty acid to toner. In this case, it is possible to improve the stability of charging effected by friction between the toner and carrier and thereby shorten a charging rise time for supplied toner.
Moreover, Japanese Unexamined Patent Publication JP A 4-124678 (1992) discloses a technique to obtain a toner in which charge exchange between the toner and carrier is made smoothly, charging can be effected at high speed, and a distribution of the amount of charge is narrow by external addition of zinc-oxide fine particles having a volume specific resistance of 100 to 108 Ωcm and an average primary particle size of 100 to 500 mμm to toner.
Further, Japanese Unexamined Patent Publication JP-A 9-325512 (1997) discloses a technique to suppress variation in the amount of charge applied to toner, to form high-quality images with ease, and to ensure high transfer efficiency by external addition of zinc oxide having an average primary particle size of 0.01 to 0.1 μm and a specific surface area of 25 to 200 m2/g to toner as an external additive.
As has already been described, it is possible to control the amount of charge applied to toner with stability by using, as an external additive, titanium oxide or zinc oxide in the form of electrically conductive particles. However, the toner disclosed in JP-A 4-452 is externally added with titanium oxide whose refractive index is higher than that of zinc oxide. In this case, as compared with a case where it is externally added with zinc oxide, the transparency of a fixed toner layer becomes lower, which leads to poor color reproducibility.
As for the toner disclosed in JP-A 4-124678, although zinc oxide whose refractive index is lower than that of titanium oxide is used as an external additive, the average primary particle size of the external additive is as large as 100 to 500 mμm. In this case, as compared with a case where zinc oxide having an average primary particle size of less than 100 mμm is used as an external additive, the transparency of a fixed toner layer becomes lower.
The toner disclosed in JP-A 9-325512 pays no regard to the size of a primary-particle aggregate of zinc oxide used as an external additive. Since zinc oxide for use has a large specific surface area, the aggregation of particles tends to occur easily. Therefore, depending on the size of a primary-particle aggregate of zinc oxide, a decline in transparency may occur.